This invention relates to electronic circuitry enclosures with ventilation holes that comply with standards regulating both EMI radiation and safety.
Air vents are needed in enclosures used for electronic circuitry so that air can be pulled into and exhausted from them for the purpose of removing the heat generated by their internal electronic components. Electromagnetic interference (EMI) emission requirements and safety requirements, however, impose major limitations on how those air vents can be implemented. The switching frequency for state-of-the-art electronic circuitry can be 2.5 GHz or higher resulting in non-negligible harmonic frequencies as high as 12.5 GHz. Holes in enclosures exceeding 0.2 inches, therefore, become one-quarter wavelength antennas and make it virtually impossible to meet current EMI emission standards.
Safety requirements also impose restrictions on the way in which air vents can be implemented. The International Electrotechnical Commission standard (IEC-950) defines a xe2x80x9cpush testxe2x80x9d designed to ensure that personnel cannot be injured by accidentally inserting appendages into the enclosure where they are exposed to a possible electric shock hazard.
It is difficult to construct an air vent with holes restricted to 0.2 inches or less that has both enough open space to allow sufficient air to pass through and enough strength to pass the IEC-950 push test. Metal sheets perforated with holes of this size tend to be either insufficiently open to air or insufficiently strong. For the same reason, screens made of fine mesh that can be relatively easily penetrated are also inadequate.
The present disclosure describes a design for a vent that simultaneously satisfies all three of these requirements: air flow, emissions containment and safety.
An air vent constructed in accordance with the principles of the invention comprises a vent assembly including an electrically-conductive screen mounted inside an enclosure constructed of panels. One or more of the panels has an array of holes through it. The holes are configured and sized to prevent an appendage, such as a finger from penetrating the enclosure. However, the collective hole area is sufficient to permit adequate airflow. The conductive screen covers the holes and provides for EMI suppression.
In one embodiment the holes have a hexagonal shape and are spaced closely together to provide adequate airflow.
In another embodiment, the conductive screen comprises one or two layers of a metal honeycomb mesh.